Circumventing Boxwood Threats with Alternative Species

The Cultivar Solution, Part 2

CIRCUMVENTING BOXWOOD THREATS WITH ALTERNATIVE SPECIES

Tim Wood, Spring Meadow Nursery

In the January/February 2020 issue of The Buckeye, I published an article titled The Cultivar Solution, which outlined the benefits of plant breeding to tame invasive species. When it comes to finding answers to the issues we face as breeders and propagators of woody ornamentals, thinking alternatively is often, if not always, necessary. Sometimes we can develop new cultivars that address emerging concerns. But just as often, existing cultivars can serve the same purpose as a threatened species, and are naturally immune to the perils they are experiencing. One such imperiled shrub is boxwood.

Because of its functionality, adaptability, and deer resistance, boxwood is one of the most utilized landscape plants in the world. Unfortunately, boxwood blight, a lethal disease caused by the fungus Calonectria pseudonaviculatum, threatens this iconic shrub. The disease, well established in Europe, has crossed the big pond and is now killing boxwood in North America.

Boxwood blight is a fungus spread by the transport of infected plants, then from infected plants to healthy plants via hedge shears, animals, and human touch. It is a slow but methodical spread that continues to widen. With that spread, plant breeders and researchers have been working to solve this problem, but it could be a very long time before it is eliminated. While blight is currently boxwood's biggest nemesis, the U.S. Department of Agriculture's Animal and Plant Health Inspection Service (APHIS) has just sent notice of a new, more lethal threat to our beloved boxwood: the box tree moth (Cydalima perspectalis). Box tree moth caterpillars feed primarily on boxwood and quickly kill boxwood plants when left unchecked. Brought to the U.S. on nursery plants shipped from Ontario, Canada, the APHIS has confirmed that the invasive pest is now present in multiple states.

Boxwood isn't having an easy time of it. Many will go to great lengths to contain this new pest and create blight-resistant cultivars, and that's important work for the survival of this popular plant. But a more immediate, and possibly one of the simplest solutions, is to use alternative shrubs with the same utility. Because these alternatives are not boxwood, they are immune to the issues that boxwood currently faces.

Two species with the greatest potential to be suitable boxwood substitutes are Ilex crenata, Japanese holly, and Ilex glabra, inkberry holly. Both have small, broad, evergreen leaves and dense branching that responds well to being sheared into hedges, globes, and spires.

Inkberry Holly

Inkberry (Ilex glabra) is an Eastern North American native evergreen that is hardy in zones 5-9. Typically, it is a large plant maturing at 5 to 6 feet in height. It grows best in moist acid soils but is adaptable to most average garden soils. Rutgers University lists it as moderately deer resistant, being seldom severely damaged by deer.

A number of cultivars on the market are more compact; however, most tend to lose their leaves on the lower portion of the plant with age. Recently, two new selections have been introduced by Proven Winners® ColorChoice® that look and behave much more like boxwood.

The first selection is called Gem Box® Ilex glabra 'SMNIGAB17'. This plant was developed at Spring Meadow Nursery in Michigan, where hundreds of seedlings were grown out and field evaluated over a ten-year period. During that time, any plants that developed bare stems were eliminated from the trial. In addition, plants that had winter burn or that were damaged by snow load were also destroyed. With time and the destruction of many plants, about a dozen individuals were selected, propagated, and trialed again both in containers and in the field. The very best of these plants was eventually chosen and introduced in the spring of 2015. The demand for Gem Box was so great that Spring Meadow had to hold off a year on shipping to build up a larger stock block. In 2016, thirty thousand plants were shipped to growers. By spring of 2018, the sales jumped to just under 100,000 plants, and a good number more could have been sold if only there had been more plants. Obviously, there is a huge demand for a good boxwood alternative.

Gem Box inkberry holly has extremely dense branching, small glossy leaves, but most importantly, it retains its lower foliage, making it an excellent replacement for boxwood. With the initial flush of growth in the spring, the foliage exhibits an attractive cast of reddish-burgundy coloration. In the spring, if you look closely, you will find that it has small white flowers, and if you have a male pollinator nearby, you will also get small black fruit later in the summer. Neither the flowers nor the fruit are very noticeable. While the plants are naturally dense and rounded in habit, they respond wonderfully to pruning and be sheared in globes or hedges with round or squared-off edges.

Later, another inkberry boxwood substitute was introduced. This selection, called Strongbox® Ilex glabra 'Ilexfarrowtracey', was developed by Mike Farrow of Maryland. We trialed this variety in our test field for over five years, and we were very impressed with its dense habit, dark green foliage, and superb winter hardiness. It has larger leaves than Gem Box and a more mounded habit, making it quite distinct. The foliage is thick and dense, even on the lower branches.

Japanese holly

Ilex crenata is also a suitable boxwood alternative and is hardy in zones 6-8. Patti-o-Box® 'FarrowSK6' and Sky Box® 'Farrowone' Japanese holly (the latter formerly sold as Sky Pointer®) are also bred by Mike Farrow and naturally grow in a convenient, columnar habit. Both cultivars have shiny, dark leaves, tight upright branching, and are well suited for containers and smaller garden spaces.

And now, for something completely different...

Juke Box® ×Pyracomeles is a remarkable new broadleaf evergreen, and it is one of my favorite boxwood alternatives. It is an intergeneric hybrid between Pyracantha and Osteomeles. It had no thorns, no flowers, and like a boxwood, can be sheared and shaped as desired. It reminds me of a 'Morris Midget' boxwood but with much faster growth. This new plant comes from Dr. Tom Ranney of North Carolina State University.

When left unpruned, it forms a thick, mounded evergreen mat, but with a bit of shearing can be formed into a ball, box, or a low hedge. Initially, I did not think this plant would be hardy in Michigan and based on the parents, I listed it as a zone 7 plant. We have now overwintered it successfully in our test field for three years, and it has not missed a beat. It is certainly hardy to zone 6 and perhaps zone 5b.

While plant pests and diseases can be disruptive, they can also force us to think creatively and can bring new opportunities. There are so many beautiful and useful plant species in the world, so we have lots of choices. When it comes to replacing boxwood, these selections fit the niche. B

Tim Wood is the New Product Development Manager at Spring Meadow Nursery. He can contacted at Tim@SpringMeadowNursery.com. Photos courtesy of Spring Meadow Nursery.